This invention relates generally to a method of unsticking a material delivery apparatus and more particularly, but not by way of limitation, to a method of changing a material delivery apparatus from a static friction state to a dynamic friction state.
A material delivery apparatus of the type referred to in this disclosure is used to move a material into a mixing unit where the material is mixed with one or more other materials into a mixture. The materials and mixture can be of any type. For example, a sand screw can be used to convey sand, referred to as proppant, into a fracturing fluid made at an oil or gas well.
When a particular amount or rate of material is to be conveyed, the material delivery apparatus is controlled either manually or automatically to obtain the desired delivery of material. At least such automatic control typically includes generating an electric control signal that causes the apparatus to operate as desired. A feedback signal is also typically used to ensure that the actual response of the material delivery apparatus to the control signal is what is desired. Use of a feedback signal creates a closed loop control system.
Although the electric control signal and the feedback signal enable automatic control to be implemented, they can also cause a runaway condition if the material delivery apparatus is stuck at start-up or becomes stuck during operation. That is, if the control signal is trying to drive the apparatus but the feedback signal indicates no operation, the automatic controller can presume that an increase in the control signal is needed. If the control signal is increased and the apparatus then becomes unstuck, the increased control signal will overdrive the apparatus whereby too much material is delivered.
The foregoing has been known to occur in prior automatic control systems with which manual methods of unsticking sand screws have been used. These manual methods include applying a vibrator to the sand screw tube to force the friction between the sand and the screw into a dynamic condition. This helps the screw to turn since the coefficient of dynamic friction is much less than the coefficient of static friction. Another method of manually unsticking a sand screw includes hammering on the tube of the screw. If all else fails, the sand screw hopper will have to be shovelled out.
A shortcoming of such manual methods is that when they are used with an automatic control system, the aforementioned runaway can result. Additionally, manual methods require a human operator to be present at the material delivery apparatus, which might present a dangerous situation. Furthermore, manual methods can be relatively slow, and they can damage the equipment.
A stuck material delivery apparatus also presents the shortcoming of failing to deliver material when it is needed, which can be highly detrimental. For example, if a sand screw is struck so that it does not deliver proppant into a fracturing fluid, and yet the deficient fluid is pumped into a well to create a fracture for enhancing oil or gas recovery, the created fracture may not be adequately propped open whereby it closes when the fracturing pressure is released. If such were to occur, the stuck sand screw would have caused material and money to be wasted because of the inadequate fracturing job. This could also prevent the well from being productive.
In view of these shortcomings, there is the need for an automatic method of unsticking a material delivery apparatus. Such a method should be capable of use with other control methods that automatically control the material delivery apparatus during normal operation so that runaway does not occur when the apparatus is automatically unstuck. Such a method should also be capable of use from and after start-up to ensure that material delivery is not stopped due to the apparatus becoming stuck.